[Amps] FL2100B QUESTIONS

[Brian Miller]

Hi All

The ZL6QH team is using an FL2100B amplifier that recently failed. Both of the Svetlana 572B tubes
had developed grid-to-filament shorts.

Closer inspection showed that the soldered connection to the anode ceramic blocking capacitor had
broken loose. The parasitic resistors were also discoloured and starting to crack. I am guessing
that some sort of high energy VHF oscillation occurred after the blocking capacitor went open
circuit.

Anyhow, we have now resoldered the capacitor connection, replaced the parasitic resistors and
installed a new set of Taylor 572B tubes.

I have just started testing using a reduced voltage HT tap on the power transformer (425V instead of
the usual 850V). There is no obvious evidence of VHF parasitic oscillations but the amplifier can be
made to oscillate in the 20, 15 and 10m bands when the input and output tank circuits are tuned to
resonance. The oscillations are only evident when the input circuit is UNTERMINATED.

Two questions

1. Have other owners of FL2100B amplifiers noticed that the amplifier has a tendency to oscillate on
the higher bands when the input circuit is unterminated? Is this "normal" for the FL2100B?

2. Have any other owners of FL2100B amplifiers experienced the same failure mechanism - blocking
capacitor open along with grid-to-filament shorts? If so, then is there anything else that we should
be watching out for?

Note. I HAVE read the earlier threads about parasitic suppression - including Richard's web pages.
No need to repeat this information please.

Thanks and 73

Brian Miller ZL1AZE
Registered Electrical Engineer
67 Hornsey Rd
Melrose
Wellington
New Zealand

Tel 027 4446614
Email: brianmiller@xtra.co.nz